Indole derivatives have been and continue to be important intermediates for dyestuffs and pharmaceuticals. Since it's discovery in the 1880's, Emil Fischer's synthetic method has been one of the most widely used methods for preparing indoles from aryl hydrazines. Various catalysts have been used to effect the cyclization of arylhydrazones derived from the reaction of aryl hydrazines and ketones/aldehydes. Bronsted acids including H2SO4, HCl, PPA, TFA, oxalic acid, formic acid, HI, HBr, propionic acid, and AcOH, Lewis acids including ZnCl2, ZnBr2, TiCl4, SnCl2, CuCl, CuBr, and PCl3, and solid acids including zeolites, and montmorillonite clay, Lewis acidic ionic liquids such as 1-butyl-pyridium chloride.3AlCl3 and choline chloride.2ZnCl2 and Bronsted acidic ionic liquids including BMImHSO4, BMImH2PO4, HMImTA, HMImBF4, HMImNO3 and HMImOTf, among others, have been used.
However, because of the complex mechanism involved, there exists high variability in the preferred conditions for specific indoles. In other words, one set of reagents and conditions does not work best for all indoles.
U.S. Pat. No. 5,179,211 describes a process of preparing indoles from phenylhydrazine and ketones in the presence of less than 5 equivalents of an acid having a pK of 1.3-4.5 and an aqueous medium. The process preferably is carried out at a temperature of 80-110° C. Preferably 2-4 equivalents of acid are used.
Liu and Robichaud (Tet Lett. 48, 461 (2007)) describe that the use of acetic acid and a temperature of 60° C. gave indolenines in good yield. Elevated temperatures led to significant side products and rearrangements.
Liu et al (Org. Lett, 8, 5769 (2006)) describe that a mixture of AcOH and MsOH also functioned in a reaction with cyclohexanecarbaldehyde and phenylhydrazine whereas ZnCl2 and H2SO4 did not perform as well. A mixture of HCl in AcOH in a reaction with isobutylaldehyde led to rearrangement to form 2,3-substituted indoles.
Edwards et al (Bio and Med Chem Lett, 8, 745 (1998)) describe the use of Fischer protocol (AcOH, 60° C.), reduction of indoles to indolenines, nitration and hydrogenation to the amino-substituted compounds.
Certain substituted indoline compounds, such as those disclosed in U.S. Pat. No. 6,995,162, including motesanib, have been found to be useful in treating conditions associated with angiogenesis, including the treatment of cancers. In addition, U.S. Pat. No. 6,878,714 describes the method of making 1-acetyl-6-amino-3,3-dimethyl-2,3-dihydroindole using reductive Heck conditions. This route generally involves the palladium-catalyzed cyclization of allylacetamide. Liu et al. (Tet Lett, 48, 2307 (2007)) describe the synthesis of substituted indolines using the Heck cyclization. The use of palladium in such reactions adds an undesired expense that would be advantageous to avoid. Thus, there is an ongoing need for more facile and higher yielding processes for preparing indoline derivatives.